Controlled Atmosphere Electrospinning of Organic Nanofibers with Improved Light Emission and Waveguiding Properties

Laser-transmission-induced Raman emission masked by progressive transparency in polymer waveguides

Laser-transmission-induced Raman emission (LTIR) in polymer waveguides is observed and analyzed in this work. When injected with a 532-nm continuous-wave laser of 10 mW, the waveguide shows a distinct line of orange-to-red emission, which is quickly masked by the green light in the waveguide due to the laser-transmission-induced transparency (LTIT) at the source wavelength. However, when a filter is applied to remove the emission below 600 nm, a clear red line is shown in the waveguide, which stays constant over time. Detailed spectral measurements show that the polymer material can generate broadband fluorescence when illuminated with the 532-nm laser. However, a distinct Raman peak at 632 nm only appears when the laser is injected into the waveguide with much higher intensity. The LTIT effect is fitted based on experimental data to describe the generation and fast masking of the inherent fluorescence and LTIR effect empirically. The principle is analyzed through the material compositions. This discovery may trigger novel on-chip wavelength-converting devices using low-cost polymer materials and compact waveguide structures.

WO3 Nanowires Enhance Molecular Alignment and Optical Anisotropy in Electrospun Nanocomposite Fibers: Implications for Hybrid Light-Emitting Systems

The molecular orientation in polymer fibers is investigated for the purpose of enhancing their optical properties through nanoscale control by nanowires mixed in electrospun solutions. A prototypical system, consisting of a conjugated polymer blended with polyvinylpyrrolidone, mixed with WO₃ nanowires, is analyzed. A critical strain rate of the electrospinning jet is determined by theoretical modeling at which point the polymer network undergoes a stretch transition in the fiber direction, resulting in a high molecular orientation that is partially retained after solidification. Nearing a nanowire boundary, local adsorption of the polymer and hydrodynamic drag further enhance the molecular orientation. These theoretical predictions are supported by polarized scanning near-field optical microscopy experiments, where the dichroic ratio of the light transmitted by the fiber provides evidence of increased orientation nearby nanowires. The addition of nanowires to enhance molecular alignment in polymer fibers might consequently enhance properties such as photoluminescence quantum yield, polarized emission, and tailored energy migration, exploitable in light-emitting photonic and optoelectronic devices and for sensing applications.

Advances in Conjugated Polymer Lasers

This paper provides a review of advances in conjugated polymer lasers. High photoluminescence efficiencies and large stimulated emission cross-sections coupled with wavelength tunability and low-cost manufacturing processes make conjugated polymers ideal laser gain materials. In recent years, conjugated polymer lasers have become an attractive research direction in the field of organic lasers and numerous breakthroughs based on conjugated polymer lasers have been made in the last decade. This paper summarizes the recent progress of the subject of laser processes employing conjugated polymers, with a focus on the photoluminescence principle and excitation radiation mechanism of conjugated polymers. Furthermore, the effect of conjugated polymer structures on the laser threshold is discussed. The most common polymer laser materials are also introduced in detail. Apart from photo-pumped conjugated polymer lasers, a direction for the future development of electro-pumped conjugated polymer lasers is proposed.

Recent Advances of the Polymer Micro/Nanofiber Fluorescence Waveguide

Subwavelength optical micro/nanofibers have several advantages, such as compact optical wave field and large specific surface area, which make them widely used as basic building blocks in the field of micro-nano optical waveguide and photonic devices. Among them, polymer micro/nanofibers are among the first choices for constructing micro-nano photonic components and miniaturized integrated optical paths, as they have good mechanical properties and tunable photonic properties. At the same time, the structures of polymer chains, aggregated structures, and artificial microstructures all have unique effects on photons. These waveguided micro/nanofibers can be made up of not only luminescent conjugated polymers, but also nonluminous matrix polymers doped with luminescent dyes (organic and inorganic luminescent particles, etc.) due to the outstanding compatibility of polymers. This paper summarizes the recent progress of the light-propagated mechanism, novel design, controllable fabrication, optical modulation, high performance, and wide applications of the polymer micro/nanofiber fluorescence waveguide. The focus is on the methods for simplifying the preparation process and modulating the waveguided photon parameters. In addition, developing new polymer materials for optical transmission and improving transmission efficiency is discussed in detail. It is proposed that the multifunctional heterojunctions based on the arrangement and combination of polymer-waveguided micro/nanofibers would be an important trend toward the construction of more novel and complex photonic devices. It is of great significance to study and optimize the optical waveguide and photonic components of polymer micro/nanofibers for the development of intelligent optical chips and miniaturized integrated optical circuits.

Versatile Approach for Reducing Propagation Loss in Wet-Electrospun Polymer Fiber Waveguides

Wet-electrospun (WES) polymer micron and submicron fibers are promising building blocks for small, flexible optical fiber devices, such as waveguides, sensors, and lasers. WES polymer fibers have an inherent cylindrical geometry similar to that of optical fibers and a relatively large aspect ratio. Furthermore, WES fibers can be produced using low-cost and low-energy manufacturing techniques with large-area fabrication and a large variety of materials. However, the high propagation loss in the fibers, which is normally on the order of tens or thousands of decibels per centimeter in the visible light region, has impeded the use of these fibers in optical fiber devices. Here, the origin of propagation losses is examined to develop a comprehensive and versatile approach to reduce these losses. The excess light scattering that occurs in fibers due to their inhomogeneous density is one of the primary factors in the propagation loss. To reduce this loss, the light transmission characteristics were investigated for single WES polymer fibers heated at different temperatures. The propagation loss was significantly reduced from 17.0 to 8.1 dB cm-1 at 533 nm wavelength, by heating the fibers above their glass transition temperature, 49.8 °C. In addition, systematic verification of the possible loss factors in the fibers confirmed that the propagation loss reduction could be attributed to the reduction of extrinsic excess scattering loss. Heating WES polymer fibers above their glass transition temperature is a versatile approach for reducing the propagation loss and should be applicable to a variety of WES fibers. This finding paves the way for low-loss WES fiber waveguides and their subsequent application in small, flexible optical fiber devices, including waveguides, sensors, and lasers.

Movement of new direction from conjugated polymer to semiconductor composite polymer nanofiber

In the past few years, there was a tremendous growth in conjugated polymer nanofibers via design of novel conjugated polymers with inorganic materials. Synthetic routes to these conjugated polymers involve new, mild polymerization techniques, which enable the formation of well-defined polymer architectures. This review provides interest in the development of novel (semi) conducting polymers, which combine both organic and inorganic blocks in one framework. Due to their ability to act as chemosensors or to detect various chemical species in environmental and biological systems, fluorescent conjugated polymers have gained great interest. Nanofibers of metal oxides and sulfides are particularly interesting in both their way of applications and fundamental research. These conjugated nanofibers operated for many applications in organic electronics, optoelectronics, and sensors. Synthesis of electrospun fibers by electrospinning technique discussed in this review is a simple method that forms conjugated polymer nanofibers. This review provides the basics of the technique and its recent advances in the formation of highly conducting and high-mobility polymer fibers towards their adoption in electronic application.

Electrospun Conjugated Polymer/Fullerene Hybrid Fibers: Photoactive Blends, Conductivity through Tunneling-AFM, Light Scattering, and Perspective for Their Use in Bulk-Heterojunction Organic Solar Cells

Hybrid conjugated polymer/fullerene filaments based on MEH-PPV/PVP/PCBM were prepared by electrospinning, and their properties were assessed by scanning electron, atomic and lateral-force, tunneling, and confocal microscopies, as well as by attenuated-total-reflection Fourier transform infrared spectroscopy, photoluminescence quantum yield, and spatially resolved fluorescence. Highlighted features include the ribbon shape of the realized fibers and the persistence of a network serving as a template for heterogeneous active layers in solar cell devices. A set of favorable characteristics is evidenced in this way in terms of homogeneous charge-transport behavior and formation of effective interfaces for diffusion and dissociation of photogenerated excitons. The interaction of the organic filaments with light, exhibiting specific light-scattering properties of the nanofibrous mat, might also contribute to spreading incident radiation across the active layers, thus potentially enhancing photovoltaic performance. This method might be applied to other electron donor-electron acceptor material systems for the fabrication of solar cell devices enhanced by nanofibrillar morphologies embedding conjugated polymers and fullerene compounds.

Synthesis of Continuous Conductive PEDOT:PSS Nanofibers by Electrospinning: A Conformal Coating for Optoelectronics

A process to synthesize continuous conducting nanofibers were developed using PEDOT:PSS as a conducting polymer and an electrospinning method. Experimental parameters were carefully explored to achieve reproducible conductive nanofibers synthesis in large quantities. In particular, relative humidity during the electrospinning process was proven to be of critical importance, as well as doping post-treatment involving glycols and alcohols. The synthesized fibers were assembled as a mat on glass substrates, forming a conductive and transparent electrode and their optoelectronic have been fully characterized. This method produces a conformable conductive and transparent coating that is well-adapted to nonplanar surfaces, having very large aspect ratio features. A demonstration of this property was made using surfaces having deep trenches and high steps, where conventional transparent conductive materials fail because of a lack of conformability.

Organic Micro/Nanoscale Lasers

Micro/nanoscale lasers that can deliver intense coherent light signals at (sub)wavelength scale have recently captured broad research interest because of their potential applications ranging from on-chip information processing to high-throughput sensing. Organic molecular materials are a promising kind of ideal platform to construct high-performance microlasers, mainly because of their superiority in abundant excited-state processes with large active cross sections for high gain emissions and flexibly assembled structures for high-quality microcavities. In recent years, ever-increasing efforts have been dedicated to developing such organic microlasers toward low threshold, multicolor output, broadband tunability, and easy integration. Therefore, it is increasingly important to summarize this research field and give deep insight into the structure-property relationships of organic microlasers to accelerate the future development. In this Account, we will review the recent advances in organic miniaturized lasers, with an emphasis on tunable laser performances based on the tailorable microcavity structures and controlled excited-state gain processes of organic materials toward integrated photonic applications. Organic π-conjugated molecules with weak intermolecular interactions readily assemble into regular nanostructures that can serve as high-quality optical microcavities for the strong confinement of photons. On the basis of rational material design, a series of optical microcavities with different structures have been controllably synthesized. These microcavity nanostructures can be endowed with effective four-level dynamic gain processes, such as excited-state intramolecular charge transfer, excited-state intramolecular proton transfer, and excimer processes, that exhibit large dipole optical transitions for strongly active gain behaviors. By tailoring these excited-state processes with molecular/crystal engineering and external stimuli, people have effectively modulated the performances of organic micro/nanolasers. Furthermore, by means of controlled assembly and tunable laser performances, efficient outcoupling of microlasers has been successfully achieved in various organic hybrid microstructures, showing considerable potential for the integrated photonic applications. This Account starts by presenting an overview of the research evolution of organic microlasers in terms of microcavity resonators and energy-level gain. Then a series of strategies to tailor the microcavity structures and excited-state dynamics of organic nanomaterials for the modulation of lasing performances are highlighted. In the following part, we introduce the construction and advanced photonic functionalities of organic-microlaser-based hybrid structures and their applications in integrated nanophotonics. Finally, we provide our outlook on the current challenges as well as the future development of organic microlasers. It is anticipated that this Account will provide inspiration for the development of miniaturized lasers with desired performances by tailoring of excited-state processes and microcavity structures toward integrated photonic applications.